6. Fixed Assets

A summary of fixed assets follows:

(Stated in millions)

	2017		2016
Land	$	428	$	479
Buildings & improvements		5,122		4,849
Machinery & equipment		32,160		33,834
Seismic vessels		103		846
		37,813		40,008
Less: Accumulated depreciation		26,237		27,187
	$	11,576	$	12,821

The estimated useful lives of Buildings & improvements are primarily 25 to 30 years. The estimated useful lives of Machinery & equipment are primarily 5 to 10 years. Seismic vessels are depreciated over periods ranging from 20 to 30 years.

Depreciation expense, which is recorded on a straight-line basis, was $2.3 billion, $2.7 billion and $3.2 billion in 2017, 2016 and 2015, respectively.